MadSci Network: Engineering


Date: Thu Mar 27 19:20:08 2008
Posted By: James Griepenburg, , Chemical consultant, Chemmet Services
Area of science: Engineering
ID: 1205254235.Eg

I do not understand your intentions and until you define them I can only 
give some basic ideas about  electrical conduction.  Electrical 
conduction requires motion of charged particles.  In metallic conductors 
these are electrons in conduction bands. In non metallic solids, liquids 
and gases the charged particles are ions.  Any material subjected to an 
appropriately large voltage can become a conductor.  Materials that 
require a minimum of voltage,such as most metals are conductors; 
materials, such as air, most glasses,crystalline solids, plastics, 
organic materials, that require very high voltages are insulaters. 
Ignoring semiconductors [] 
there are materials that are intermediate in conduction; These are ionic 
compounds in which the ions are mobilized by either melting, such as 
molten silicate glasses or molten salts, or by dissolution in a polar 
solvent such as salts in water, or by dissociation as in the self 
protonation of water, alcohols, ammonia, or the disassociation of strong 
and weak acids in polar solvents such as water.

Conduction of ions in solution has 5 requirements:  1. mobile ions in 
solution;  2. a site for oxidation [anode] and a site for reduction 
[cathode]; 3. a cathode half reaction and an anode half reaction.  4. a 
conductive pathway between the anode and cathode sites; and 5. an 
electrical potential difference between the anode and cathode.  If one is 
lacking conduction ceases.  The electrical potential  can be chemical for 
galvanic reactions or imposed by an electric current for electrolytic 
reactions.  This is explained in the following reference and in many 
chemistry texts.


Water would be an excellent insulator except for its self ionization into 
hydronium [H3O+] and hydroxide  [OH-] ions that are present in even the 
purest water; add common contaminants and water is a fairly good 
conductor.  Most fairly simple electrical devices that are scrupulously 
cleaned of ionic contaminants will operate for a short while if placed in 
high quality deionized water.  But under the voltages present corrosion 
slowly occurs, ions build up, alternate electrical pathways develop, and 
the device fails, sometimes quite dramatically.

The above reference lists some common galvanic potentials, check it out, 
and they are all under about 5 volts so electrolytic reactions don't need 
much voltage to happen, anything over fractions of a volt can cause 
something to happen.  So the normal approach to protect against 
electrolytic failure is to remove the ionic pathway by good insulation 
and encapsulation or by sealing the device or submerging in an insulating 
fluid.  I think PCBs were used for that.

Of course there is another approach and we are fortunate in that each of 
us has a [usually] fully functioning computer that operates submerged in 
water  OUR BRAIN.  I am not really knowledgeable in its function but we 
can intuit several things 1. it operates at very low voltages,  The 
cathode and anode reactions are tuned  to operate at these voltages and 
probably rely on monitoring the electrolyte concentrations as switches, 
there must be appropriate ion permeable membranes to control the 
electrical potentials, the presence of contaminants must be strictly 
controlled for proper operation.  There are plenty more so read up on it.

Back to your idea of conductive plastics in water. the usual failure mode 
is electrochemical destruction of a metal conductor and formation of 
metallic conductive pathways between areas of differing voltage.  Plastic 
conductors would eliminate this by removing anode and cathode reactions.  
Conductive polymers tend to be only fair conductors so any reasonable 
answer would probably be in the nanotechnology carbon nanotube area.  The 
device would have to be massively parallel and highly integrated. 

Here is an idea of what is happening.  Go on the web and to the library 
and see what can be done and make a simple prototype. Happy inventing, JimG

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